This is VERY true. the 5GHz on my netgear has better range as my 2.4GHz linksys did, but the linksys was single-band and the netgear is dual or tri band - (more ways to connect and less multipath echos) Higher freq = smaller wave = less propagation.
So, speaking strictly range, your BEST bet is to get a 802.11b router in 1-2MB/s mode, as it will give the MAXIMUM range possible as it has the largest wavelength. http://www.medicalel...medical-devices
paragraph below fig 1. specifically states this. All in all this is a good read too.
Im thinking that multipath is the problem in the OP setup, and signal propagation is more of an issue than actual coverage.. 5GHz will not (likely) remedy this problem. It will only fix interference issues and allow double the throughput.
Maybe go buy $600 worth of routers (dlink, netgear and linksys) and test them for the 14 days or whatever the return policy is.. see which works better and return the other 2.
this is a good PDF, they test propagation through things and plot it. like blinds, carpet, glass, brick, water, nearly everything. In the abstract they derive -6.8dB is mathematically calculated gain from a jump of 2.4 to 5GHz (in laymens terms, if 2.4GHz is 100% strength you'd get 25% strength with 5GHz, as every 3dB loss is halfing the power)
Now - according to their tests, the experienced loss is about equal on both bands.... This means the propagation loss experienced is the same, so signal range would be about the same. Maybe the linksys isn't all that bad afterall.
Kinda went on a tangent but out of this, if there is a lot of full water piping running between floors - 2.4GHz will be severely affected. This is also why microwaves use this frequency, as it excites water and heats up our food (and also explains why things without water in them won't heat right)
I doubt you have the gear to test how well signals propagate, so i'd just try routers and if they don't work or you don't like, return them until you either proved the top of the line ones aren't any better than what you got or you found one that works. FWIW, find one you like, return it, then order for likely cheaper online.